How Amazon customers forced the retailer to fix the greatest uncertainty of buying clothes online

Sure, Amazon sells dresses, sunglasses, and purses — but it mostly just sells socks.Facebook/Amazon FashionAmazon is a clothing powerhouse — just not the way you might imagine.

Though the company is hoping to become the go-to shop for everything high fashion, it mostly sells practical, unglamorous items like work pants and socks.

Men's and women's undergarments, including socks, accounting for a total of $415 million in sales, were two out of five of Amazon's top-selling apparel categories last year, according to a report by One Click Retail.

Rounding out the top five: men's work clothes, women's denim, and the largest, men's pants and shorts.

The top five individual apparel items sold on Amazon last year were also all either pants or socks, according to the report.

What do most of these categories have in common? They aren't the kind of item you need to try on before you buy them. Socks are one-size-fits-all, underwear is lettered sizes, and in pants, denim, and workwear you likely know what size you'll be and how they'll look without even looking at the garment.

They're all items you know you're going to use and need without having to be concerned about returns. The categories are also male-dominated — a group that possibly cares less about how clothing fits and is more hesitant to enter traditional clothing stores.

It makes sense, then, why Amazon needed to introduce a service to unlock the rest of the apparel sector — those tricky items that are returned more often for sizing or because they just look a little different in person.

Amazon, with its new Prime Wardrobe program, has made it much easier to send unwanted clothing back to the retail giant. Returns are a big part of online shopping. Nearly half — 48% — of customers surveyed by retail data firm Nazar say they returned an online purchase in the last year

Amazon Fashion hasn't broken the high-end apparel sector — yet.Facebook/Amazon FashionThe program allows Prime users to pick out several items of clothing to be sent, free of charge, in a special box that is designed to be resealable. A free UPS label is included, and customers are only charged for the clothing they keep.

Unlike most clothing stores, Amazon has a key disadvantage in that there are no physical locations to return clothing items to easily. With Prime Wardrobe, Amazon can finally unlock the hardest sector of apparel to sell online and take over the fashion industry.